Conduct studies on respiratory syncytial virus (RSV) infection in an intact experimental animal (ferrets and/or cotton rats) which develops disease similar to that seen in young infants hospitalized with RSV bronchiolitis or pneumonia. The purpose of the study is to define the factors which are important in lower respiratory tract disease produced by this virus. The study should include the role of immunologic factors in resistance to RSV infection and disease as well as their possible participation in the immunopathologic potentiation of disease. Conduct studies on RSV vaccines or antiviral compounds in the animal model at the direction of the Project Officer.